1. Field of the Invention
The present invention relates to a liquid crystal display device, in particularly, to a liquid crystal display device having alignment layers used for alignment control.
2. Description of the Related Art
In the prior art liquid crystal display devices using active elements, a TN (twisted nematic) mode liquid crystal has been mainly used, wherein a liquid crystal having a positive anisotropy of dielectric constant is inserted between a pair of substrates so that liquid crystal molecules are aligned generally in the horizontal direction relative to the surface of the respective substrate, and the alignment direction of the liquid crystal molecules is twisted at 90 degrees between the opposed substrates. This TN mode liquid crystal, however, has a drawback in that a good viewing angle is small.
To improve the viewing angle characteristic, a VA (vertically aligned) mode liquid crystal is proposed in place of the TN mode liquid crystal, wherein a liquid crystal having a negative anisotropy of its dielectric constant is inserted between a pair of substrates so that liquid crystal molecules are aligned generally in the vertical direction relative to the surface of the respective substrate.
In this VA mode, however, it is necessary to carry out an alignment division to eliminate the reversal of brightness at a middle tone. As one method for carrying out the alignment division, there is a newly developed alignment method using irradiation with ultraviolet light. This alignment method using irradiation with ultraviolet light has an advantage in that the alignment direction can be controlled in a non-contact manner with the alignment layer. According to the method for controlling the alignment direction by irradiation with ultraviolet light, however, there is a problem in that the pretilt angle becomes excessively small and results in a risk of reduction of contrast.
On the other hand, another type of liquid crystal display device has been proposed recently wherein a ferroelectric or anti-ferroelectric liquid crystal is used to realize a liquid crystal mode having a wider viewing angle and a quick response characteristic one thousand times faster than that of the TN mode liquid crystal.
There is a problem in this liquid crystal display device in that, because the liquid crystal material used therefor has a higher alignment property, it is very difficult to control the pretilt angle which is necessary for preventing defects of alignment from being generated, by solely relying on the capacity for generating a pretilt of the alignment layers on its own. This is one of reasons for the difficulty of applying the alignment method using irradiation with ultraviolet light.
FIGS. 21 to 23 illustrate the prior art and the problems thereof.
FIG. 21 illustrates a prior art liquid crystal panel using a ferroelectric liquid crystal, and FIG. 22 illustrates problems in the liquid crystal panel shown in FIG. 21.
As shown in FIG. 21, a liquid crystal panel 300 is composed of a pair of substrates 80 and 81 having electrodes 82 and 83 and alignment layers 84 and 85 of PVCi (polyvinyl cinnamate) formed on the inner sides thereof, respectively. A ferroelectric liquid crystal layer 86 is inserted between the pair of substrates 80 and 81. This liquid crystal panel 300 is referred to as a chevron structure of a ferroelectric liquid crystal wherein liquid crystal molecules 86a are disposed in a structure in consecutive bending layers.
According to the prior art liquid crystal panel 300, however, the liquid crystal molecules 86a are aligned generally in the horizontal direction relative to the surface of the substrates as shown in FIG. 21 to have a pretilt angle of generally zero degree, whereby there may be zones wherein the bending direction of the layered structure is reversed to result in a zigzag-alignment defect. FIG. 22 shows the zigzag-alignment defect including a hair pin defect 87 and a lightning defect 88 which deteriorates the display quality.
FIG. 23 illustrates another prior art liquid crystal panel wherein a VA mode liquid crystal is used. As shown in FIG. 23, a liquid crystal panel 400 is composed of a pair of substrates 90 and 91 having electrodes 92 and 93 and vertical alignment layers 94 and 95 of polyimide or polyamic acid on the inner sides thereof, respectively. The alignment treatment of the vertical alignment membrane 94 and 95 may be carried out by irradiation with ultraviolet light. Between the pair of substrates 90 and 91 is inserted a liquid crystal layer 96 formed of a liquid crystal having a negative anisotropy of dielectric constant.
In this liquid crystal panel 400, since the alignment treatment of the vertical alignment layers 94 and 95 is carried out by irradiation of ultraviolet ray, a pretilt angle xcex8p of the liquid crystal molecule 96a relative to the surface of the substrate is considerably reduced. This results in more leakage of light to decrease contrast.
Also, under such circumstances, the range of selection of materials for the alignment layers to which an alignment treatment by irradiation with ultraviolet light can applied might be limited, and the production cost rises.
The object of the present invention is to solve the problems in the alignment occurring due to the magnitude of a pretilt angle and to provide a liquid crystal display device having an excellent display quality.
A liquid crystal display device according to the present invention comprises a pair of substrates, a nematic liquid crystal layer inserted between the pair of substrates, an alignment control layer provided on the inner side of at least one of the substrates, the alignment control layer having a saw-tooth cross-section with slopes inclinated at an angle relative to the substrate, and an alignment layer covering the alignment control layer.
In this arrangement, in the liquid crystal display using a nematic liquid crystal, alignment of the liquid crystal can be controlled by the alignment control layer and the alignment layer, irrespective of kinds of materials or alignment treatment for the alignment layers, to realize a desired pretilt angle.